Barrier films are used in a wide variety of technologies, including medical and commercial devices. For example, barrier films find use in short and long term implantable medical devices, seals, gaskets, blood contact surfaces, bags, containers, and fabric liners. In addition to good barrier properties, barrier films should have good mechanical properties and be thermally stable. Monolithic, multi-component, and multilayered barrier films have been constructed as barrier materials, but have not provided a combination of thermal stability, strength, and barrier properties.
Polytetrafluoroethylene (PTFE) has been evaluated for use as barrier films. The use of PTFE is advantageous in that it can be used in harsh chemical environments and over a broad range of temperatures. For example, PTFE has exhibited utility as a material for use in harsh chemical environments where other polymers quickly degrade. PTFE also has a useful temperature range from as high as about 260° C. to as low about −273° C. However, PTFE barrier films are characterized by poor mechanical properties such as low tensile strength, poor cold flow resistance or creep resistance, poor cut-through and abrasion resistance, and a general poor mechanical integrity that precludes its consideration in many materials engineering applications.
Low porosity PTFE articles have been made through the use of a skiving process in which solid PTFE films are split or shaved from a thicker preformed article. These PTFE articles are characterized by low strength, poor cold flow resistance, and poor load bearing capabilities in both the length and width directions of the film. Processes such as ram extrusion of PTFE fine powder have also been used to produce low porosity PTFE articles; however, such films also possess relatively poor mechanical characteristics. Attempts have also been made to strengthen the low porosity PTFE films by stretching in the length dimension. However, strength gains are minimal and, by the nature of the process, are achieved in only a single dimension, thus greatly minimizing the utility of the film.
An expanded polytetrafluoroethylene (ePTFE) film may be produced by a process taught in U.S. Pat. No. 3,953,566, to Gore. The porous ePTFE formed by the process has a microstructure of nodes interconnected by fibrils, demonstrates higher strength than unexpanded PTFE, and retains the chemical inertness and wide useful temperature range of unexpanded PTFE. However, such an expanded PTFE film is porous and therefore cannot be used as a barrier layer to low surface tension fluids since such fluids with surface tensions less than 50 dyne-cm pass through the pores of the membrane.
Compressed ePTFE articles in which a platen press was used to densify a thin sheet of ePTFE with and without heat are also taught in U.S. Pat. No. 3,953,566 to Gore. However, cold flow occurred in the press, non-uniform parts resulted, and a density of over 2.1 g/cc was not achieved. Accordingly, the utility of the ePTFE sheet as a barrier film was limited.
Conventional processes for forming TFE-based barrier films involve expansion, compression, and subsequent thermal treatment with or without deformation. Additionally, high strength dense PTFE barrier films may be produced without the use of the expansion and compression processes by the deformation of dried PTFE paste at a temperature above the crystalline melt of PTFE. Although such processes may result in a high strength dense fluoropolymer film, crystallinity is greatly reduced, permeability is not optimized, and the process is limited in scale.
Thus, there exists a need in the art for a TFE-based barrier film that demonstrates improved barrier performance, such as evidenced by a resistance to methane permeation, improved physical and mechanical performance, such as low creep, and high matrix tensile strength, as well as a simplified process for making the TFE-based barrier film.